Automatic frequency control circuits



0. E. KEALL AUTOMATIC FREQUENCY CONTROL CIRCUITS March 26, 1940.

IJCIPlM/NHRIR Filed Oct. 22, 1937 lNVENTDR OSWOLD E. KEALL QFC 8/195 ATTORNEY Patented Mar. 26, 1940 UNITED STATES AENT OFFICE AUTOMATIC FREQUENCY CONTROL CIRCUITS Oswold Edward Keall, Chelmsford, England, assignor to Radio Corporation of America, a

corporation of Delaware Application October 22, 1937, Serial No. 170,343 In Great Britain December 31, 1936 8 Claims. (01. 250 -40) it 10 determined range on either side of the correct tuning. position. A. F. C. systems-are sometimes (rather loosely) termed automatic tuning systerns, since a. measure of automatic tuning is obtained thereby, although, in fact, the A. F; C.

system provides merely accuracy of tuning, selection as between stationsbeing efi'ected by other means, usually an ordinary manually controlled tuning knob.

It is known that if the anode-cathode spaceof go a valve is in efifect, included in a tuned circuit, and there is fed to the control grid of saidvalve an. alternating voltage substantially in quadraerally, in the known arrangement, of this nature produced by means of two series impedances in and the other resistive, Such an arrangement, however, ofiers the defect that the valve throws not only reactance, but also resistance (which is always positive) across the tuned circuit and this.

4 resistance is normally of suificient magnitude to of a superheterodyne receiver. (a common case for A. F. C.) the magnitude of the resistance 45 thrown across the tuned circuit by the valve will generally be suflicient to involve alteration of the design of the local oscillator circuit. of which the tuned circuit forms, part. This invention provides improved circuit arrangements operating 50 on the principle just described but not involving the defect just mentioned and which are such that if desired the valve may be caused to act as a substantially pure reactance as regards the tuned circuit, although, if desired, the valve may be caused to throw resistance (which may be parallel with the tuned circuit, one being reactive lead to difficulties; for example, where the tuned circuit is required to be that of the local oscillator.

either positive or negative) across said circuit. The invention also provides circuit arrangements which though not free from producing resistance changes as Well as reactance changes, have been found to fulfill practical requirements, inmost 5 cases. i

The object of the invention is achieved by deriving the quadrature voltage for the gridof the valve from a portion of a phase splittingnetwork constituting a substantially constant 1111- 10 pedance and preferably constituted by two branches in parallel, one branch consisting of an inductance in series with a resistance and the other consisting of a condenser in series with a resistance. I g

The invention is illustrated in and further ex plained with reference to the accompanying circuit, diagrams wherein each of Figs. 1, 2 and 3 show different modifications of the invention.

, Referring to: Figure 1. which showsdiagrammatically one way of carrying out this invention 'as applied to asuperheterodyne receiver, the local oscillator of said receiver comprises a valve i for example, a back coup-led triode as shown-- having the usual manually tunable circuit 2, 3, in I its anode-cathode circuit. Shuntedacross the anode-cathode space of the local oscillator valve l is the anode-cathode space of a control valve 4 which is shown as avariable mu-pentode whose control grid-cathode circuit includes the secondary 5 or a transformerfif desired in series with a capacity-shunted bias resistance combination 6, 1. Coupled to the coil 2 in the local oscillator tuned circuit 2, 3, is a second coil 8 which is. included between control grid and cathode of a further valve 9 which is shown as a variable mu-pentode in whose anode circuit is included a phase splitting circuit comprising two branches in parallel, one branch consisting of an inductance ill in series with a resistance-ii and the 40 7 other consisting of a condenser ii in series with aresistance l3. The saidcondenser and the said inductance have a common point i5 which is connected to the anode i5 ofthe valve 9 the common point [6 of the two resistances being eon- 4,5 nected through the usual anode battery (not shown) to the cathode l'lof said valve 9. The

control grid-cathode circuit of the valve 9 may, also include a capacity shunted bias resistance combination I9, 20. The primary 2! of the trans- I0 and resistance H in one "branch of the phase i splitting circuit and at the other to the junction 23 of the condenser i2 and resistance iiin the other branch. The phase splitting circuit is a substantially constant impedance circuit whose resistance is equal to the square root of the quotient obtained by dividing its inductance by its capacity, the resistances H, is being equal and the reactances iii, i2, being so dimensioned that the voltage across the branch I2, i3, is equal to and in quadrature with the voltage across the branch iii, 5 each of these voltages being at 45 phase angle to the voltage between the points 22, 23.

With the above arrangement the alternating voltage applied to the grid of the control valve It will in the ideal case be a quadrature with the local oscillator tuned circuit voltage and the said control valve will act as though it were a substantially pure reactance across the local oscillator tank circuit 23, the said reactance being either an inductance or a capacity in dependence upon the sense of'the coupling between the local oscillator tuned circuit 2, 3, and the'grid circuit of the further valve 9. This reactance may be varied over a wide range by varying the mutual conductance of the further valve 9 and/or of the control valve 4, e. g., by varying the bias on one or both of these valves. In practice, this variation is obtained automatically in dependence upon departures of the local oscillator tuning from accuracy and is such as to correct for such departures. The means for automatically producing such variation do not per se, form part of this invention and anyof the numerous known means may be employed; e. g., use may be made, as known per se, for A. F. C. purposes, of two sharply tuned circuits one resonant slightly above and the other slightly (and the same amount) below, the correct predetermined intermediate frequency, these circuits being fed from an intermediate stage and feeding into asuitable rectifier arrangement so that, if the intermediate frequency actually produced is above the predetermined value a correcting bias which produces variation in one direction is obtained, while if the actually produced intermediate frequency is lower than it should be, a correcting voltage which produces variation in the other direction is obtained. I

By departing slightly from the quadrature condition of the alternating input to the control valve, the said valve may be caused to throw either positive or negative resistance across the local oscillator circuit, the sign of the resistance depending upon the direction of departure. If desired, this action may be made adjustable by utilizing an adjustable condenser for the condenser H. in the phase splitting circuit in the anode circuit of the valve 9. By suitably interlinking such an adjustable condenser with the normal tuning control knob as shown by dotted line 3', the oscillator amplitude may be caused to remain substantially constant over the tuning range or it may be caused to increase or desistance into the local oscillator circuit may be of substantial advantage.

Although, as above set forth, the valve may be caused to throw resistance into the tuned circuit as well as reactance, in general, reactance only will be required.

By suitably choosing the value of the impedance of the constant impedance phase splitting circuit, relatively large changes of reactance pre sented by the valve may be obtained per unit change of mutual conductance of the valve.

In the modification shown in Figure 2, the further valve 9 of Figure l is dispensed with and instead there is provided, in shunt with the anode-cathode space of the local oscillator valve I, a circuit'comprising a low reactance condenser 24 (connected to the anodes of the local oscillator and control valves l and i) in series with a high resistance 25 which is in turn in series with a phase splitting circuit as shown in Figure 1. The primary 25 of the screened transformer 2 l-5 is connected to points in the two branches of the phase splitting circuit, just as in Figure 1. To give a practical example, experimentally tested, the high resistance was about 50,000 ohms and the impedance (constant) of the phase splitting circuit about 1,0G ohms. With these values a change of 9 volts bias on the control valve (which, for this example, was a pentode) produced an eiTective change of 4 micro-micro-farad across the local oscillator.

Figure 3 shows a modification of the circuit of Figure 2. The difference between Figure 3 and Figure 2 is that in the said Figure 3. the transformer winding 5 is the secondary of an auto-transformer whose primary is constituted by the coil Iii in the phase splitting circuit. In the theoretically ideal case, the voltage across the whole circuit consisting of elements 25, 25, l2, H, H! and M will be in phase with that across the network l2, H, H, H), and both these voltages will be in quadrature with that between the grd .of the valve i and the point I6. This, however, neglects the eifects of mutual inductance and leakage.

In all three circuits, the relative position of the elements It, H, and also of the elements l2,

l3, may be reversed if, for practical reasons, this may be found desirable. Again, in all cases, the ideal constant impedance condition-namely that resistance elements H and 53 are equal to one another in value and each equal to the square root of the quotient obtained by dividing the value of the inductance it by that of the capacity l2 may be departed from to some extent if, in any particular case, a predetermined resistance variation accompanying reactance variation is required.

By employing a suitable double purpose valve e. g., a triode-pentodein lieu of the two valves in the embodiments of Figures 2 and 3 still further simplification can be achieved.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. In combination with a local oscillator provided with a resonant tank circuit. a frequency control tube having said tank circuit included in its space current path, a phase splitting network coupled to said tank circuit and feeding voltage derived from the latter to the input electrodes of the control tube, said network comprising pair of parallel reactive arms of opposite ign, the voltages developed across arms being equal and in phase quadrature, voltage fed to said input electrodes being substan ially in quadrature with the voltage across tank circuit.

2. In combination with a local oscillator provided with a resonant tank circuit, a frequency control tube having said tank circuit included in its space current path, a phase splitting network coupled to said tank circuit and feeding voltage,

derived from the latter to the input electrodes of the control tube, said network comprising parallel reactive branches, one branch including capacity andt'he other branch including inductance,- alternating voltages developed across said arms beingequal and in phase quadrature, said voltage fed to said input electrodes being substantially in quadrature with the voltage across the tank circuit.

3. In combination with a local oscillator provided with a resonant tank circuit, a frequency control tube having said tank circuit included in its space current path, a phase splitting net work coupled to said tank circuit and feeding voltage derivedfrom the latter to the input electrodes of the control tube, said voltage fed to said input electrodes being substantially in quadrature with the voltage across the tank circuit, said network comprising a pair of parallelbranches,

derived from the latter to the input electrodes of the control tube, said network comprising a pair of parallel reactive arms of opposite sign, the

, voltages developed across the arms being equal and in phase quadrature, said voltage fed to said input electrodes being substantially in quadrature with the voltage across the tank circuit, a second tube having said network in the anode circuit thereof, means coupling saidtank circuit to the input electrodes of the second tube.

5. In combination with a local oscillatorprovided with a resonant tank circuit, afrequency control tube having said tank circuit included in its space current path, a phase splitting network coupled to said tank'circuitand feeding voltage derived from the latter to the input electrodes of the control tube, said network comprising a pairof parallel reactive arms of opposite sign, the voltages developed across the arms being equal and in phase quadrature, said voltage fed to said input electrodes being substantially in quadrature with the voltage across the tank circuit, and

means for varying the gain of the control tube thereby to adjust the value of reactance simulated acrosssaid tank circuit by said control tube.

6. In combination, with the resonant circuit of an oscillator network, means for tuning the cir-' cuit over a' desired frequency range, a frequency control arrangement comprising a control tube having input and output electrodes, said resonant circuit being connected to the output electrodes of the control tube, a network coupling the resonant circuit to the control tube input electrodes, said 7 network comprising a pair of reactive arms of opposite sign and applying voltage derived from across said resonant circuit to said input electrode but in substantially quadrature relation thereto, means. for adjusting the magnitude of one of said arms with adjustment of the tuning means, and means for varying the gain of said control tube.

7. In combination the. resonant circuit of an oscillator network, a frequency control arrangement comprising a control tube having input and output electrodes, said resonant circuit being connected to the output electrodes of the control tube, a network coupling the resonant circuit to, the control tube input electrodes, said network comprising a pair of reactive arms of opposite sign and applying voltage derived from across said resonant circuit to said input electrode but in substantially quadrature relation thereto, the

reactive values of said arms being so chosen that alternating voltages developed thereacross are equal and in phase quadrature, said network beinginclucled in a shunt circuit connected across said resonant circuit, and means for varying the gain of said control tube.

8. In combination with a tunable oscillatory circuit which is adapted to cover a relatively wide "frequency range, means for supplementally adjusting the tuning of the circuit at desired frequencies of the range, said means comprising an electron discharge device having at least a cathode, control grid and anode electrode, means connecting the cathode to anode impedance of the device across said oscillatory. circuit, aphase splitting network comprising at least two reactive branches connected across the said circuit, said network being constructed to develop thereacross an alternating voltage in quadrature with i the alternating voltage across said circuit, means for impressing the quadrature voltage between the grid and cathode, and the constants of the said network being chosen to maintain the impedance thereof substantially constant over said range.

OSWOLD EDWARD KEALL. 

